To describe how Registered Nurses make use of a Clinical Decision Support System to triage calls to emergency medical dispatch centres, from the perspective of professional autonomy. The study had a descriptive design with a qualitative inductive approach. Interviews were done with 24 Registered Nurses during 2018-2019. Thematic analysis was conducted. Five themes and 16 subthemes were established: (a) Using the CDSS as a general support to professional competence in emergency calls, including subthemes: Support for professional competence, an aid to reflection, a compulsory support; (b) A specific support useful in difficult situations and calls, with subthemes: RN being tired or stressed out; vague and unclear symptoms, rare situations, aggressive and agitated callers; (c) Using the CDSS but changing triage recommendations/priority, including subthemes: Recommending a higher priority than the CDSS and recommending a lower priority than the CDSS; (d) Development areas for better use of the CDSS in collaboration with other services, with subthemes: Request for common documentation system with ambulances and closer collaboration with the national telephone nursing helpline; and (e) Possible technical development areas in the CDSS for optimal use, including subthemes: image transfer, medical records, development of certain areas in the CDSS, update of maps, a need for more knowledge. The CDSS was not perceived as a restriction on professional autonomy. It was particularly useful in rare situations. Technical improvements as well as education and training should be done in close collaboration with registered nurses. The study contributes with knowledge about how registered nurses triaging emergency calls use a decision support system. The system was a support for professional competence and did not seem to restrict them. The findings could be useful for clinicians and researchers in development of telephone triage and decision support systems.